With the popularity of portable electronic devices, the switching power supply becomes a main power solution for various electronic devices.
Referring to FIG. 1, in a conventional technology, a conventional switching power supply 10 mainly includes: a primary winding module 101, a primary controller 102, and a primary current sampling loop 103 coupled with a current sampling port CS of the primary controller 102, a DC (direct current) power output stage circuit 104 coupled with a secondary side of a transformer T1 and a power supply & voltage feedback loop 105 of a power supply terminal VDD of the primary controller 102 coupled with an auxiliary winding Na of the transformer T1. The primary winding module 101 receives an input signal VIN of the switching power supply 10, and one terminal of an input capacitor Cin receives the input signal VIN and the other terminal is grounded. The primary winding module 101 includes a clamping circuit 1011, and a primary winding Np of the transformer T1 coupled with the clamping circuit 1011. The primary controller 102 includes a PWM (Pulse Width Modulation) module 1021 (the PMW shown in FIG. 1), a controller 1022 coupled with the PWM module 1021 and a power switch transistor M1. The primary current sampling loop 103 includes a resistor R5. The DC output stage circuit 104 includes a secondary winding Ns of the transformer T1, a diode D2 coupled with a dotted terminal of the secondary winding Ns, an output capacitor Cout, a load 1041 and a resistor Rout. The power supply and voltage feedback loop 105 includes the auxiliary winding Na, and the auxiliary winding Na has a dotted terminal coupled with a feedback voltage division resistor R4 and a positive pole of a diode D1. The power supply a voltage feedback loop 105 further includes a feedback voltage division resistor R7, a starting resistor R6 and a capacitor Cvdd. The starting resistor R6 receives the input signal VIN of the switching power supply 10. A feedback terminal FB of the primary controller 102 is coupled with the feedback voltage division resistor R4 and the feedback voltage division resistor R7.
In the conventional switch power supply 10, when a voltage of the input signal VIN of the switching power supply 10 is higher than a withstand voltage of the power switch transistor M1, there is a burning risk of the power switch transistor M1 when the power switch transistor M1 is conducted. When the voltage of the input signal VIN is lower than a rated voltage threshold of the normal operation, a driving capability of a circuit system is insufficient, and an output voltage ripple of the circuit system may not meet a standard specification, which may cause the load 1041 to fail to work properly.